Natural Selection
Resumen
Look closely at any wild population and no two members of it are quite the same. One runs a little faster. One is a shade darker. One survives a cold night that kills its neighbours. Those differences look too small to matter until you notice what the surroundings are doing with them, quietly, generation after generation: keeping some, discarding others, and never once consulting the organisms about it. Given enough generations that sorting rebuilds a species. Given a strong enough pressure it can do it inside a few years, which is why a poison that worked last decade may not work now.
This lesson takes you through the mechanism doing the rebuilding, and it is a short mechanism: four steps that fit on one line and are worth four marks when they arrive in the right order. You will see why an individual organism never evolves although a population does, why a weightlifter's children are not born strong while a beetle's colour is inherited, how a population of rats can stop responding to a poison without a single rat ever changing, and how one species split in half by a stretch of sea can drift so far apart that the two halves can no longer breed together at all. Everything in this topic is Core Tier content, which means every word of it is examinable whichever tier you sit, and the specimen papers make that point more forcefully than the specification does.
Objetivos
- Theories of how organisms have evolved include: the theory of evolution by natural selection; other theories, including that of Lamarck, are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited. We now know that in the vast majority of cases this type of inheritance cannot occur.
- Evolution occurs via natural selection.; Individual organisms within a particular species may show a wide range of variation because of differences in their genes.; Individuals with characteristics most suited to the environment are more likely to survive to breed successfully.; The genes that have enabled these individuals to survive are then passed on to the next generation. Students should develop an understanding of the time scales involved in evolution.
- New species arise as a result of: isolation: two populations of a species become separated, eg geographically; genetic variation: each population has a wide range of alleles that control their characteristics; natural selection: in each population, the alleles that control the characteristics which help the organism to survive are selected; speciation: the populations become so different that successful interbreeding leading to fertile offspring, is no longer possible.
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Nota de la lección
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- Which statement best describes the theory associated with Lamarck? A. Changes that occur in an organism during its lifetime can be inherited B. Individuals best suited to the environment are more likely to survive to breed C. New versions of genes are produced by random mutation D. Populations become separate species when they are isolated Answer: A
- In natural selection, where does the variation between individuals originally come from? A. From individuals practising a useful skill during their lifetime B. From differences in their genes, with new versions arising by random mutation C. From the environment altering the genes in adult body cells D. From organisms developing the characteristics they need Answer: B
- Two isolated populations of one species are considered to have become separate species when they can no longer: A. Live in the same habitat B. Look similar to one another C. Interbreed to produce fertile offspring D. Compete for the same food Answer: C
- A field of insect pests is sprayed with an insecticide. A few insects survive and breed. What is the best explanation? A. The insecticide caused those insects to mutate B. Those insects learned to avoid the spray C. Those insects already carried a version of a gene giving resistance D. The insects that died had used up their resistance Answer: C
- Which sequence gives the correct order of the stages by which a new species arises? A. Speciation, isolation, natural selection, genetic variation B. Isolation, genetic variation, natural selection, speciation C. Genetic variation, speciation, isolation, natural selection D. Natural selection, speciation, genetic variation, isolation Answer: B
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